3G Evolution. Outline. Chapter: Multi-antenna configurations. Introduction. Introduction. Multi-antenna techniques. Multiple receiver antennas, SIMO

Transcription

1 Chapter: 3G Evolution 6 Outline Introduction Multi-antenna configurations Multi-antenna t techniques Vanja Plicanic vanja.plicanic@eit.lth.se lth Multi-antenna techniques Multiple transmitter antennas, MISO Multiple antennas at both RX and TX, MIMO Department of Electrical and Information Technology 1 2 Introduction Multi-antenna configurations Multi-antenna systems Base station (BS) User Equipment (UE), ex. Mobile station (MS) Multi-antenna techniques Smart antennas Single-input single-output Single-input multiple-output Multiple antennas at the receiver and/or transmitter + Smart signal processing Multiple-input single-output Multiple-input single-output 3 4

2 Antenna configurations cont. Antenna configurations cont. - Configuration of the antennas is decided by the requirement on the antenna mutual coupling and correlation (low/high) - Thus, configuration decided by choice of - spatial distance between the antennas - However, the case of MS at low frequencies < 900 MHz => 0.5 wavelengths is large distance for low frequencies Common phone size allows for ~0.25 wavelengths distance at 850 MHz! Low mutual coupling and correlation when: BS: >10 wavelengths (due to small AoA in macro cell, shorter distance in micro cells) MS: >0.5 wavelengths (due to wide AoA) - polarization directions of the antennas Antennas with different polarizations for both BS and MS gives i lower mutual l coupling and correlation. => Polarization diversity hard to implement due to antenna + chassis radiation, difficult to rotate chassis wave-mode Multi-antenna techniques cont. Multi-antenna techniques cont. Why? How? - To improve system capacity (more users per cell), better link reliability DIVERSITY DIVERSITY - Antennas at receiver and/or transmitter - Mitigates fading in the radio channel - Low mutual coupling required - To improve coverage (possibility for larger cells) BEAM-FORMING BEAM-FORMING - Antennas at receiver and/or transmitter - Shaping of antenna beams to maximize gain in certain direction or suppress specific interferer - Low or high mutual coupling required - To achieve higher data rates per user, higher spectral efficiency SPATIAL MULTIPLEXING SPATIAL MULTIPLEXING - Antennas at both receiver and transmitter - Sending several data streams on multiple parallel channels - Low mutual coupling required Figures above Courtesy of Ericsson 7 Figures above Courtesy of Ericsson 8

3 RX diversity - aims to: mitigate fading suppress specific interferer Linear receiver antenna combining Smart signal processing techniques: - RX diversity - Receive beam-forming - Adaptive space time processing - All information is exploited by combining copies of the signal from all the antennas (in comparison to switched/selection diversity) - Assumes non-time variant channel - Weights the signal copies with corresponding amplitude and phase correction - Noise limited system: - Maximum Ratio Combining (MRC) - Interference limited system: - Maximum Ratio Combining (MRC) - Interference Rejection Combining (IRC) - Minimum Mean Square Error (MMSE) RX diversity Linear receiver antenna combining in: - Noise limited it case: - Maximum Ratio Combining (MRC) - Interference limited it system: RX diversity Maximum Ratio Combining (MRC) - Amplitude and phase weighting - Phase weights- adjustment to assure that signals from two antennas are aligned - Amplitude weights- adjustment of the received signals to correspond to the channels gain, higher weight for stronger signals. - Diversity gain and array gain - For noise limited environments - Maximum Ratio Combining (MRC) - Interference Rejection Combining (IRC) - Minimum Mean Square Error (MMSE)

4 RX diversity Interference Rejection Combining (IRC) - For interference limited environment - Uplink intra-cell interference suppression, Spatial Division Multiple Access (SDMA) - Able to suppress Nr-1 R interferers, however large noise increment after combining Adaptive space-time processing - Frequency selective channel - Linear time domain filtering/equalization, linear processing to signals received at different times (MRC, Zero-forcing, MMSE) - Linear receive antenna combining, linear processing to signals received at different antennas Minimum Mean Square Error (MMSE) - Weights to minimize the difference between the estimated and transmitted signal. Receive beam-forming Receive beam-forming - Switched beam antennas - Antenna array that can form pattern beams pointing in certain discrete direction -switching selects the best beam for down conversion and post processing, goal to maximize the SNR - simple implementation, since only one signal to post process - limited flexibility, since only fixed directions Switched antenna array Adaptive antenna array - Amplitude and phase weights MRC => a receiver beam with maximum gain N R in the direction of the target signal IRC => a receiver beam with high attenuation in the direction of the target signal Figures above Courtesy of jackwinters.com

5 TX diversity - Does not require channel knowledge at the receiver - Techniques: - Delay/Temporal diversity - Cyclic-delay diversity - Space time/frequency coding (STBC/STFC) Smart signal processing techniques: - TX diversity - Transmit beam-forming TX diversity Delay/Temporal diversity - Time variant channel TX diversity Cyclic-delay diversity - Applies cyclic shift hiftinstead of flinear delays => signals received at different times are uncorrelated => delay diversity already there and can be extracted in advanced receivers (ex. GRAKE) - Time in-variant channel => create artificial time dispersion (frequency selectivity) => transmit identical signals with different delays from different antennas - Delay diversity usually implemented by forward error correction, ARQ, repetition coding etc. - Delay diversity invisible to mobile terminal since it is just additional time dispersion handled by the receiver

6 TX Diversity TX Diversity Space-time block coding (STBC) Space-frequency block coding (SFBC) - Sending same but differently coded information on each of the antennas, ex. Alamouti scheme - Space-frequency Transmit Diversity (SFTD) - used in 3G WCDMA standard as Space Time Transmit Diversity (STTD) - Orthogonal STBC => full rate=1, full diversity gain only for two antennas - No array gain, only diversity - Space-time trellis to provide full diversity, array gain and coding gain Multiple antennas at both RX and TX Transmit beam-forming - Requires channel knowledge - Antenna configurations with high mutual coupling - Small antenna distances - Different phase shifts applied to steer the direction of the beam - CLASSICAL BEAM-FORMING - High array gain, no diversity gain - Antenna configurations with low mutual coupling - Large antenna distances or different polarization - Different gain and phase shifts to steer the direction of the beam - Pre-coding decided from: - Channel feedback from mobile terminal average downlink estimate, ex. FDD - Recommendation from mobile terminal - Pre-coding for non-frequency-selective fading and white noise - Maximum Ratio Transmission - instant channel estimate, fast beam-forming - diversity gain and array gain - Pre-coding for frequency-selective fading not possible, NB! OFDM time invariant sub-channels Smart signal processing techniques: - Spatial multiplexing - Pre-coder based spatial multiplexing

7 Multiple antennas at both RX and TX Multiple antennas at both RX and TX Spatial multiplexing Spatial multiplexing Background: - SIMO and MISO Low SNR => capacity increase ~ SNR increase (N T xn R ) High SNR => capacity increase ~ log2(snr) - Spatial multiplexing Thus, capacity increase ~ min {N T, N R } Multiple antennas at both RX and TX Multiple antennas at both RX and TX Pre-coder based spatial multiplexing Some SM detection techniques - If SNR low => beam-forming better than spatial multiplexing - If N L =number of multiplexed streams = N T Pre-coding=> orthogonalizes parallel streams, better signal isolation - If N L < N T => combination of beam-forming and spatial multiplexing used - Maximum-Likelihood ML - Layered space time architectures res (BLAST) - Successive Interference Cancellation (SIC) - Single and Multi-codeword Transmission - Per Antenna Rate Control (PARC) - Depending on the channel information pre-coder code-books chosen

8 Chapter summary References Multi-antenna techniques SIMO Diversity gain and array gain MISO Diversity gain and/or array gain MIMO Diversity gain and/or array gain and/or multiplexing gain [1] Dahlman E. et al., 3G evolution-hspa and LTE for Mobile Broadband, 2nd edition, Elsevier, UK 2008 [2] Paulraj A. et al., Introduction to Space-Time Wireless Communications, Cambridge, UK 2003 [3] Molisch A.F., Wireless Communications, IEEE Press, Wiley & Sons, US 2006 Multiplexing gain Diversity gain Diversity gain and array gain Spectral efficiency Link reliability Coverage 29 30